Agriculture Reference
In-Depth Information
heated; (ii) decrease the radiation losses
from the canopy and its surroundings; and
(iii) limit the energy losses due to air leak-
age and convection, provided they are
properly installed and they are not perme-
able, so that the enclosure is fairly
airtight.
Thermal screens are used at night, in
winter and in cold areas. The energy sav-
ings range from 5% with calm weather
and overcast sky, to more than 60% with
strong winds and clear sky (Urban, 1997a).
The main problem they pose is the
increase of the air humidity, if they are
not permeable.
The screens can be made of linen or
similar material, that may be woven, or
plastic films, and aluminium strips are often
present to improve efficiency (Photo 7.2).
The strips have low emissivity of radiation,
are highly reflective and do not transmit
radiation. If they are used only at night, they
do not need to be transmissive to solar radi-
ation. Linens have very variable optical
properties. The plastic films are very air-
tight. It is of fundamental importance to
assemble screens properly in order to
achieve maximum efficiency (for details see
Chapter 4). They must be installed so that
once they are folded shadows are mini-
mized, because they limit light by at least
4% (Hanan, 1998).
The management of screens consists
of extending them at dusk and folding
them back at dawn. Folding must be pro-
gressive to avoid the sudden fall of the
cold air mass (accumulated over the
screen) over the plants. If they reduce
radiation (due to being folded back late or
extended early) they will limit photosyn-
thesis, negatively affecting the yield.
Their extension and retraction can be
automated so that these occur at a pre-
fixed time or when radiation reaches a
certain threshold.
7.3.3
External double sidewalls
The insulation of the external sidewalls
around the perimeter of the greenhouse is
an efficient measure to limit heat losses.
The thermal behaviour is similar to that of
inflated walls. It is simple and cheap, result-
ing in little light reduction. The most com-
monly used material is a plastic film with
air pockets.
Photo 7.2.
Thermal screen.
